1. Field of the Invention
The present invention is directed generally to a three-dimensional scan converter (3D SC) for use in 3D ultrasound imaging, and specifically to a 3D SC apparatus that converts the input 3D data in linear-linear, sector-linear, sector-sector and linear-sector scan formats to a cubic rasterized data matrix.
2. Background of the Invention
Two-dimensional ultrasound imaging has been widely used in cardiology, radiology and other clinical diagnostic areas for more than thirty years, since J. J. Wild and J. M. Reid published their first paper of ultrasound medical B-scan imaging in Science in 1952. As an example, two-dimensional echocardiography offered considerable advantages over M-mode echocardiography because of the ability to provide real-time tomographic images of the heart, and it extended the ability of practitioners to make complex diagnostic decisions. In the beginning, there were questions by some whether two-dimensional approaches were worth the tithe and expense. Continued experience with these methods provided an opportunity for clinicians to ask new questions. Although similar questions have been directed to three-dimensional ultrasound imaging for years, three-dimensional echography appears to be a desirable goal as it would likely provide a method for deriving new anatomical and functional indices of the human heart and other organs.
Early efforts on the investigation of three-dimensional ultrasound imaging can be tracked back to later 50's to earlier 70's. But the results on 3-D medical ultrasound imaging, such as, ultrasound transmission tomography and holography, were unfortunately disappointing because of poor image quality caused by the diffraction effect, sound speed variations in tissues and the penetration limitation of sound in some tissues and organs, such as bone and lung. Data acquisition and image reconstruction speed was too slow to provide any meaningful and practical clinical use. Even though the investigation never stopped, the public interest in 3-D ultrasound imaging kept a low profile for a long time.
Following recent advances in computer technology and high-speed digital electronics interest in three-dimensional medical ultrasound imaging has gradually recovered. High speed 3-D imaging systems with heavy parallel processing have been built to supply as fast as 15 volume-per-second real-time imaging frame rate, even though the image quality is still very poor compared with the image standard for the up-to-date two-dimensional B-scan. The clinical applications of ultrasound three-dimensional imaging have been reported in many areas, from cardiology to ophthalmology.
The benefits to be obtained from current 3-D ultrasound imaging system are still debated because of the low frame rate and low signal-to-noise ratio. Both these existing problems could be overcome in the future with the advance of technology. It is clear that there are strong, and gradually increasing clinical needs for this type of imaging modality. Vast literature has been published in recent years on the clinical applications of the ultrasound three-dimensional imaging technique.
In cardiology, despite the obvious advantages of two-dimensional echocardiography methods over M-mode, serious limitations remain. It is important to obtain a short axis B-mode sector scan of the left ventricle of the heart. But it is limited to some extent by the acoustic windows offered by the rib cage and the lungs. With a three dimensional imaging system, a C-scan plane can be walked vertically through the sector, providing short axis view at any desired level, from the apex, through the center of the ventricle, through to the level of the mitral valve and beyond. In some patients, such a variety of scans would not be available to the physician with a standard two-dimensional system, because of the restrictions of the acoustic window.
Cardiac structures are spatially complex and a mental picture of the heart must be acquired from a series of two-dimensional interrogations. For example, calculation of ventricular volume by echocardiography must be performed based upon complex geometric assumptions such as whether a ventricle is elliptical or not. In the setting of a severe regional wall-motion abnormality, the ventricle may not conform to any geometric assumption and the resultant quantitative volume information is, therefore, limited.
Likewise, quantitative assessment of wall motion data derived from two-dimensional echocardiography is subject to limitations imposed by the complex spatial forward and rotational movements of the heart between diasrole and systole. Almost all such computer based models are, therefore, inherently limited because it becomes spatially impossible to determine the same geometric center of the ventricle between diastole and systole for reliable determination of wall-motion indices.
In addition, the normal breast is characterized by a well-ordered spatial organization of the connective and glandular tissues. 3-D reconstruction allows surface analysis of the tumor smooth envelope, and is clearly distinct from the normal parenchyma. The adenocarcinoma has an irregular, jagged envelope, with poor limitations from the surrounding tissue. 3-D will lead to significant increases in specificity and sensitivity for breast tumor diagnosis with ultrasound and to better comprehension of cancer-dystrophy relationships. 3-D will also lead to progress in antenatal diagnosis through spatial visualization of fetal organs, and allow the development of new diagnostic and therapeutic procedures in utero.
Interventional Cardiology has grown very rapidly in recent years. Ultrasound imaging catheter has been used for thrombus and stenosis diagnostics during PTCA procedures. Because of lacking three-dimensional spatial scanning capability, current image interpretation is relying on physician's experience and still of manual catheter manipulation. In recent years, several prototype 3-D imaging catheters have been presented. By off-line processing, series 2-D image sections scanned in different depths are stacked together to reconstruct a three-dimensional coronary artery image. This 3-D image not only speeds the diagnostic process, but also increases the diagnostic efficacy by providing physicians the ability to view the diseased artery segment in all directions most of which are not accessible with conventional diagnostic techniques. Some of the prototypes even can open and flat the artery segment to let physicians inspect the inner wall of the artery which has very important information for the diagnosis.
During the past two decades, medicine has benefited from a trend toward minimally invasive procedures. Miniature surgical devices are being developed that can be introduced through small incisions to perform many elective surgical procedures. Many of these procedures will require ultrasonic imaging techniques for guidance. In some situations, three-dimensional scanning capability, with automatic localization and display of the scan plane containing an interventional device, would help by eliminating the need to manually track the interventional device.
The three-dimensional ultrasound imaging and related tissue characterization methods have been applied fruitfully in a number of medical applications, including evaluation of intraocular tumors, cancers of the liver, clots and thrombi, skin lesions and prostate tissue.
More clinical applications of the three-dimensional ultrasound imaging will be discovered after the imaging system is commercially available and more physicians get familiar with this imaging technology.
Ultrasound's low cost, noninvasive, and repeatable way of capturing dynamic images have led to the widespread success of two-dimensional B-scan imaging. These benefits should apply equally for three-dimensional ultrasound imaging. Current 2-D imaging system can obtain sector, linear and other formats of real-time images in a frame rate greater than 30 frames per second. Image detail resolution has been pushed almost to the ultrasound diffraction resolution which is the ultimate limit in this type of imaging methodology. The contrast resolution has reached the negative 70 dB level. There is still a huge gap between the current two-dimensional image quality and the three-dimensional image quality which can be realistically achieved in the near future.
One major technical obstacle is how to convert 3D ultrasound scan formats, such as sector-sector, to regularized raster cubic data matrix for image processing and CRT display. Due to the huge amount of data received from 3D scan, the 3D scan conversion could take a long time if using conventional computation technique. This is not acceptable for real-time clinical requirements.